With recent development of electronic devices, color imaging, high image quality and high imaging speed are required in the art of image formation by electrophotography, and there have been various developments in technology for achieving colorization, particle size reduction and ensured fixing ability in toner, and ensured stability in toner and developer against environmental variation, and so on.
For example, Japanese Unexamined Patent Publication No. HEI 11 (1999)-272016 (Patent Document 1) discloses an electrophotographic toner mainly containing a binding resin, a colorant, an azo metal compound and a triphenylmethane compound, and having a volume resistivity of 3×1010 to 15×1010 Ωcm, wherein the content ratio by weight of the azo metal compound to the triphenylmethane compound is 3:97 to 97:3.
Thereby, an electrophotographic toner which can be applied to copying machines and the like including whichever of positively and negatively charged photoreceptors and which maintains stable tribocharge properties over a long period of time is provided, and a sufficient image density and stable image properties with less background fogging are achieved.
In addition, Japanese Unexamined Patent Publication No. 2005-316306 (Patent Document 2) discloses an electrophotographic two-component developer including toner particles containing a binding resin, a colorant and a charge controlling agent; and a silicon resin-coated ferrite-based carrier, wherein the colorant content is 10% by weight or more with respect to the total amount of the toner particles, the toner particles have a specific volume resistance of 20×109 to 85×109 Ωcm, and the ferrite-based carrier has a resistance of 2.0×1010 to 1.0×1012 Ωcm when a 500-V DC electric field is applied by a bridge method at a distance of 6.5 mm.
Thereby, an electrophotographic two-component developer is provided which allows toner therein to be given a sufficient amount of charge so that high-quality images can be steadily formed even in image formation under a high-temperature and high-humidity environment, image formation from a document having a so high coverage that the image density exceeds 35%, or the like.